More research needed for TCD in California walnuts

At least 60 cases of thousand cankers disease are confirmed in California; about half each in commercial English walnut orchards and native black walnut trees in residential and riparian areas.
TCD occurs after the walnut twig beetle carries spores of the fungus Geosmithia morbida on the insect’s body into the tree where it bores holes in the tree bark and disrupts the phloem tissue.

The swing of the ax at state and federal budgets continues to chip away funds to battle agricultural pests and diseases in the West including thousand cankers disease (TCD) in walnut (Juglans) species in California.

At least 60 cases of the vector-transmitted disease are confirmed in California; about half each in commercial English walnut orchards and native black walnut trees in residential and riparian areas, says Richard Bostock, UC Davis plant pathologist.

“Thousand cankers is an example of new and emerging diseases which require diagnostic and research expertise to develop effective disease management options,” Bostock said.

Research dollars are limited to learn more about TCD in California walnut species. Bostock and his colleagues received funds from the USDA to evaluate various walnut (Juglans) species, including English walnut, for differences in susceptibility and resistance to TCD.

Bostock is collaborating with USDA Forest Service entomologist Steven Seybold of the Pacific Southwest Research Station in Davis, Calif., to understand the biology of TCD, particularly as the disease develops in California.

TCD occurs after the walnut twig beetle (WTB), Pityophthorus juglandis, carries spores of the fungus Geosmithiamorbida on the insect’s body into the tree. The tiny insect bores holes in the tree bark to the phloem tissue. Numerous entry holes by beetles into the bark and subsequent fungal cankers are the reason for the term thousand cankers.

The phloem is the tree’s major pathway for carbohydrate movement. During colonization, the fungus kills the phloem and cankers form around the beetle galleries. Male beetles generate pheromone which attracts females that in turn create galleries to lay eggs. As the phloem degrades, bleeding cankers can become visible on the external bark.

Early TCD symptoms include yellowed leaves and thinning foliage in the tree’s upper crown followed by large branch dieback and tree collapse. Often there is bleeding on the branches and trunk. The disease has been confirmed on several black walnut species, English walnut, and Paradox and black walnut rootstocks.

There is no effective control for TCD. The disease is only found in walnuts. Still unknown is how the beetle acquired TCD.

“I don’t think there will be a magic bullet to solve the TCD issue,” Bostock said. “Insect-vectored canker diseases in trees are very difficult to manage. Our hope is TCD will not become a significant problem in English walnut orchards and California native black walnuts. However, the experience elsewhere in the West in plantings of eastern black walnut (Juglans nigra) is that the disease can aggressively kill trees.”

TCD and the association of the fungus and the insect vector were first identified though studies of dying eastern black walnut in Colorado by plant pathologist Ned Tisserat and entomologist Whitney Cranshaw of Colorado State University.

In California, native black walnuts (Juglans hindsii) infected with TCD were first confirmed in Yolo County in June 2008 and one month later in Solano County by the Colorado scientists, Seybold, and UC Davis researchers. Counties with confirmed cases in English walnut include Sutter, Yuba, Tulare, Yolo, Solano, Stanislaus, Fresno, and San Benito.

Threat to English walnut industry

TCD is a potential threat to California’s commercial English walnut industry.

California farmers grow 99 percent of the nation’s English walnut crop. According to the California Department of Food and Agriculture, the 2009 value of the walnut industry totaled $738 million. The top walnut-producing counties (in order) include San Joaquin, Butte, Tulare, Stanislaus, Tulare, and Sutter. Growers harvested about 227,000 acres of walnuts in 2009.

TCD is found in native black walnut trees from Los Angeles in the south to Sutter and Lake counties in the north. Outside of California, TCD is found in black walnuts in Arizona, Oregon, Washington, Utah, Idaho, Colorado, New Mexico, and Tennessee.

Janine Hasey, UC Cooperative Extension tree crop farm advisor for Sutter and Yuba counties, first found TCD in English walnuts in Sutter County in August 2009 in the Howard variety.

“The disease can kill a tree in two to three years,” Hasey said.

Symptomatic bleeding from the beetle holes can resemble symptoms from other walnut ailments including shallow bark canker and Phytophthora root rot.

Hasey and Steve Seybold collaborated in TCD research in 2009 including yellow sticky traps placed on a dying black walnut tree in Sutter County to monitor WTB flight and landing rates. Beetle captures occurred from early mid-April to late October. The highest catches occurred in late June and August.

Hasey and Seybold found WTB adults and larvae in the dying tree trunk in early April which suggests the insect overwintered in the tree.

This year, the researchers are conducting a chemical insecticide and fungicide trial on black walnut. The trial is designed to prevent the WTB from entering the bark and increase tree resistance to fungal infection prior to the beetle entering the tree. They are also testing an experimental pheromone trap.

Tested chemicals include the longer residual insecticides Brigade and Sevin. Brigade is a longer residual pyrethroid. Sevin is commonly used to treat bark beetles in forests.

“We are looking mainly at prevention which would require several sprays,” Hasey said. “Once the beetle enters the bark, fungal control is difficult due to canker formation by the fungus.”

Hasey has since found the disease in more Howards and Franquette varieties. The disease was found only in declining trees from age or other problems.

TCD management: detection

Most of the TCD confirmed finds in northern California are in black walnut so far. The disease has been confirmed in about a dozen English trees. Surveys are needed to determine the extent of TCD in English walnut.

“The only management for TCD right now is detection,” Hasey said. “Growers need to be aware of this disease but not be alarmed.”

If TCD is suspected in an English walnut tree, Hasey says contact the local UCCE farm advisor. E-mail a digital photo of the tree to the advisor. Hasey notes that sudden branch death during the summer can be the result of branch wilt, not TCD.

For trees confirmed with TCD, Hasey recommends removal if the entire tree is diseased. In partially diseased trees, remove the upper branches to the green wood.

Burn infected wood on site according to local restrictions. Chipping wood will not kill the bug but will interfere with the insect’s egg-laying cycle. Do not sell infected wood for firewood or woodworking.

Hasey says TCD is not a quarantine issue since then disease is widely found.

In Tulare County, UCCE farm advisor Elizabeth Fichtner first found TCD in English walnuts in Fall 2009. To date, at least 10 cases are confirmed in English countywide. Fichtner has found TCD in the varieties Tulare, Chico, and Chandler, and on Paradox and black walnut rootstocks.

“I was in an orchard looking at trees for a different problem, turned around, and saw it,” Fichtner said. “It was very dramatic. The tree had all of the stereotypical TCD characteristics.”

Fichtner has found the disease in mature English trees, not in new plantings.

Unlike Hasey’s finding of the disease only in declining trees, Fichtner has found TCB in healthy English walnut trees. Hasey and Fichtner have confirmed cases on English walnut with and without bleeding.

Additional research is needed to learn more about the disease, vector, and fungus.

“Funding is tight right now in the research realm with a lot of financial cutbacks in Washington, D.C.,” Bostock said.

Bostock is a plant pathology professor at UC Davis and serves as executive director of the National Plant Diagnostic Network (NPDN). The NPDN is an award-winning program established in 2002 and funded by USDA to enhance agricultural security through training of first detectors and diagnosticians, and support for diagnostic laboratories.

The NPDN seeks to create infrastructure and processes to enable the rapid detection and identification of new and emerging pest problems in crops before they can become established or widespread.

“Unfortunately, the NPDN budget has been cut by 39 percent for 2011,” Bostock said. “One of the diseases we are concerned with nationally is thousand cankers disease.”

Bostock says funds are needed to better understand the biology of the pathogen and the insect in all walnut species. Down the road, resistance development through walnut breeding may offer the best possible solution.